PEER-REVIEWED ARTICLE bioresources · thermoplastic (or many thermoset) polymers are hydrophobic. As a result of this divergent behavior, the adhesion between plant fibers and polymers

PEER-REVIEWED ARTICLE bioresources.com

Özmen et al. (2013). “Effect of Acetylation on WPCs,” BioResources 8(1), 753-767. 753

Effect of Wood Acetylation with Vinyl Acetate and Acetic Anhydride on the Properties of Wood-Plastic Composites

Nilgül Özmen,a Nihat Sami Çetin,

a* Fatih Mengeloğlu,

a Emre Birinci,

b and

Kadir Karakuşa

Chemical modifications of Scots pine (Pinus sylvestris) wood flour were performed with vinyl acetate (VA) and acetic anhydride (AA) in the presence of potassium carbonate as a catalyst. Scots pine wood flour samples were successfully acetylated with VA (19 wt% gain) and AA (24 wt% gain). The effect of chemical modification of the Scots pine wood flour with AA and VA on the mechanical properties of wood high-density polyethylene composites (WPC) was determined. It was observed that acetylation of wood flour allowed a significant increase in both the mechanical properties and the thermal stability of the WPCs. It was concluded that acetylation of lignocellulosic fibers improves thermal stability, dispersion in the polymer matrix, and compatibility with the polymer matrix.

Keywords: Acetylation; Acetic anhydrides; Vinyl acetate; Wood plastic composites; Scots pine

Contact information: a: Department of Forest Industry Engineering, Faculty of Forestry,

Kahramanmaraş Sütçü İmam University, Kahramanmaraş, Turkey; b: Araç Vocational School of Higher

Education, Kastamonu University, Kastamonu, Turkey; *Corresponding author: [email protected]

INTRODUCTION

A composite material can be defined as the heterogeneous combination of two or

more components to form a filled or reinforced matrix in which the combined effect of

the components possesses superior properties compared with the individual components

alone (Popa and Breaban 1995). Replacing man-made fiber with a lignocellulosic

alternative offers a relatively cheaper and more environmentally friendly alternative

material. The most conspicuous problem of using lignocellulosic fiber as reinforcement

in a synthetic matrix is the lack of compatibility of the lignocellulosic material with the

matrix. Lignocellulosic materials are hydrophilic (abundance of OH functionality), while

thermoplastic (or many thermoset) polymers are hydrophobic. As a result of this

divergent behavior, the adhesion between plant fibers and polymers can be very poor.

The bonding between the reinforcing material and the matrix plays an important role in

determining the mechanical properties of a composite material.

Various methods may be employed to improve bond quality or compatibility of

lignocellulosic materials with a hydrophobic matrix. Two methods are the use of

compatabilizers (silanes, isocyanates, MAPE (maleic anhydride-modified polyethylene,

etc.)) and chemical modification of the lignocellulosic material (acetylation, epoxide

modification, etc.) (Schneider and Brebner 1985; Maldas et al. 1988; 1989; Kokta et al.

1989; Karnani et al. 1997; Rowell 2006a).

For improving the bond between the matrix and the reinforcing material, coupling

agents such as isocyanates and silanes (Raj et al. 1989; Maldas et al. 1989) or

mailto:[email protected]



compatabilizers can be used because compatabilizers possess properties of both the

matrix and the reinforcement in one molecule (Maldas et al. 1988; 1989; Felix and

Gatenholm 1991; Rowell et al. 1997; Sanadi et al. 1993; 1997; Simonsen et al. 1997;

Oksman and Clemons 1998; Oksman and Lindberg 1998).

Chemical modification of wood or natural fibers can be defined as a chemical

reaction between some reactive part of a wood or fiber component and a single chemical

reagent, with or without a catalyst (Rowell 2006b). All of the methods investigated to

date have involved the chemical reaction between a reagent and the cell wall polymer

hydroxyl groups. A number of reviews of chemical modification of lignocellulosic

materials have been published (Banks and Lawther 1994; Kumar 1994; Matsuda 1996;

Hon 1996; Rowell 2006a; Hill 2006). Most of the chemical modification reactions are

carried out in order to improve the dimensional stability or bio-resistance of wood. Many

wood modification reactions have been studied, of which the most useful are those that

include reaction with epoxides, anhydrides, or isocyanates (Hon 1996; Rowell 2006b;

Çetin 2000; Çetin and Özmen 2001; Çetin et al. 2005; Hill 2006; Özmen et al. 2007).

Maritime pine, Scots pine sapwood, and hemp fibers have recently been successfully

acetylated with a new transesterification reaction using vinyl acetate in the presence of

various catalysts such as potassium acetate, potassium carbonate, and sodium carbonate

(Jebrane and Sebe 2007; Jebrane et al. 2011; Çetin et al. 2011; Özmen 2012). In the

literature, there are some papers dealing with acetic anhydride modified fiber reinforced

polymer composites (Tserki et al. 2006; Lisperguer et al. 2007; Abdul Khalil et al. 2007;

Luz et al. 2008; Mat Taib et al. 2010). Some authors claim that acetylation can improve

the mechanical and thermal properties of WPCs (Tserki et al. 2006; Lisperguer et al.

2007; Abdul Khalil et al. 2007). In contradiction to these studies, it was also reported that

acetylation reduced the mechanical properties of WPCs (Luz et al. 2008; Mat Taib et al.

2010).

The objective of this study was to determine the mechanical performance of

acetylated and unmodified Scots pine wood flour-reinforced high density polyethylene

composites. Two different approaches of acetylation (acetic anhydride and vinyl acetate)

were evaluated and the results were compared with those of unmodified WPCs.

EXPERIMENTAL Preparation of Wood Flour

For wood modification, Scots pine (Pinus sylvestris) sapwood was ground using a

hammer mill and sieved to a size of 250 to 425 m. Before reaction, the samples were

subjected to Soxhlet extraction with deionised water for 6 h, and then with a mixture of

toluene/acetone/ethanol (4/1/1, v/v) for 6 h. This was done to remove the extractive

materials from the wood. The samples were then oven-dried at 103±2°C and allowed to

cool to ambient temperature in a desiccator containing phosphorous pentoxide before

determination of weight.

Acetylation of Scots Pine Wood Flour 500 g of the extractive-free Scots pine sapwood flour was transferred to a 2-L

glass reactor containing a dimethyl formamide (DMF) solution with acetic anhydride

(AA) or vinyl acetate (VA). All reactions were performed with the presence of potassium

carbonate (K2CO3) as a catalyst at 100C for 6 h. The acetylation reaction conditions



were derived from an earlier publication (Çetin et al. 2011). The treated samples were

Soxhlet extracted with deionised water for 6 hours then with the toluene/acetone/ethanol

mixture (4/1/1, v/v) for 6 h. This was done to remove excess unreacted chemicals and

byproducts. The acetylated Scots pine wood flour was oven-dried overnight at 103±2°C,

transferred to a desiccator containing phosphorus pentoxide until cool, and then weight

gain levels (WPG) were determined according to Equation 1,

100(%)

1

12x

W

WWWPG

(1)

where W1 is the sample weight before treatment and W2 is the sample weight after

treatment.

Infrared Spectroscopy Infrared absorption spectra of the acetylated and unmodified wood flour were

obtained using the KBr (potassium bromide) technique with a Shimatzu 8400s FT-IR

spectrometer. The spectra were collected over the 4000 to 400 cm-1

wavenumber range,

at a resolution of 4 cm-1

(40 scans).

13C CP-MAS NMR Analysis

Solid state 13

C CP-MAS (Cross Polarization-Magic Angle Spinning) NMR

spectra of acetylated and unmodified Scots pine wood flour were obtained at room

temperature using a Bruker DPX-400 NMR spectrometer (Bruker), with MAS rates of 4

and 8 kHz and a frequency of 100.61 MHz for 13

C NMR. Acquisition time was 0.07 s.

The pulse repetition time was 4 s and the cross polarization contact time was 1.5 s. All

the spectra were run for 15 h (25,000 scans).

Thermogravimetric (TGA) Analysis The thermogravimetric analyses of the control and acetylated wood flour-

reinforced HDPE composites were performed using a thermal gravimetric analyzer

(Perkin Elmer STA-6000). The sample weight was 20 mg. The measurements were

carried out at a heating rate of 20C/min in an inert nitrogen atmosphere with a flow rate

of 20 mL/min. Weight loss versus temperature were recorded.

Production of Wood-Plastic Composites (WPC)

High-density polyethylene was obtained from Petkim, Turkey (HDPE; Petilen

YYS0464). Four different composites were produced: HDPE (neat polymer), W20 (20

wt% extractive free wood flour filled HDPE), AA20 (20 wt% acetic anhydride modified

wood flour filled HDPE), and VA20 (20 wt% vinyl acetate modified wood flour filled

HDPE). Initially, the wood flour and HDPE were mixed with a high-speed mixer for 10

min then WPC pellets were produced using a single screw extruder. The temperature

setting from the hopper to die was 150/170/170/177C, and the screw speed was 100

rpm. The pellets were dried at 70C overnight. For injection moulding, the pellets

(wood/HDPE ratio of 20/80 by weight) were melted by a single screw HDX-88 Injection

Molding Machine (pressure: 100 bar, injection speed: 80 mm/sec; screw speed: 40 rpm).

Then the molten material was injected into dumbbell-shaped moulds to produce the



samples for tensile strength testing and into two bar-shape moulds to produce the flexural

and impact specimens. After injection, the samples were rapidly cooled.

Mechanical tests were performed in order to determine the mechanical properties

of the produced WPCs. Prior to testing, all samples were conditioned at a relative

humidity of 65±5% and a temperature of 23±2C for 7 days. For each WPC group, ten

replicates were tested. Tensile specimens (dogbone shape (type III)) were tested with a

crosshead speed of 5 mm/min. The flexural specimens (5 mm (t) x 13 mm (w) x 150 mm

(l)) were tested in the three-point loading mode with a crosshead speed of 2 mm/min on a

Zwick Roell Z010 Universal Testing machine. Notched impact specimens (5 mm (t) x

12.7 mm (w) x 64 mm (l)) were tested on a Zwick HIT5.5P Impact Testing machine. The

notches were cut using a Polytest notching cutter by RayRan. The density, tensile,

flexural, and impact properties of the manufactured samples were determined in accor-

dance with ASTM D792 (2007), ASTM D638 (2001), ASTM D6109 (2005), and ASTM

D256 (2005), respectively.

Scanning Electron Microscope (SEM) Analysis The WPC bars were dipped in liquid nitrogen and then snapped in two to get

fractured surfaces for SEM analysis. The surfaces were analyzed directly with a JEOL

scanning electron microscope (Model NeoScope JCM-5000).

Statistical Analysis Analysis of variance (ANOVA) was used to determine the effect of acetylation on

the selected mechanical properties using Design-Expert 7.0.3. The resulting F value was

compared to the tabular F value at the 95% probability level. When F tests resulted in

significant differences, comparisons between means were made by Tukey test.

Statistically different groups (P0.05) are shown on the graphs with different capital

letters.

RESULTS AND DISCUSSION

The reaction mechanism between Scots pine wood flour hydroxyl groups and

acetic anhydride (AA) or vinyl acetate (VA) is shown in Fig. 1. In order to compare the

classical AA modification with the new VA technique, the same reaction protocols were

applied to both methods. The 24% and 19% WPG levels were achieved for 6 hours of

reaction with K2CO3-catalyzed acetic anhydride (AA) and vinyl acetate (VA), respect-

tively.

Figure 2 shows the FTIR spectra of AA-modified (24% WPG), VA-modified

(19% WPG), and unmodified (W) Scots pine wood flour. As can be seen in Fig. 3, there

was no spectral difference observed between the acetic anhydride-modified and vinyl

acetate-modified samples. All modified samples showed a strong carbonyl stretching

vibration (C=O) peak in the region of 1745 cm-1

. Both the AA and VA-modified

samples also showed a peak in the region of 1242 cm-1

due to the C-O stretching

vibration of the acetyl moieties. The esterification reaction was also confirmed by the

appearance of a new peak at 1375 cm-1

, attributed to the C-H bending (s C-H) vibrations

of the methyl groups introduced (Çetin et al. 2005, 2011; Özmen 2012).



Fig. 1. The reaction of (a) acetic anhydride and (b) vinyl acetate with Scots pine wood flour

Fig. 2. FTIR spectra of untreated (W), acetic anhydride-modified (24% WPG) (WAA), and vinyl acetate-modified (19% WPG) (WVA) Scots pine wood flour

For further confirmation, the same samples were also characterized using 13

C CP-

MAS NMR. The results are shown in Fig. 3. The carbon atoms of the cellulose

component are assigned to the following peaks: C1 (anomeric C) 105 (ppm), C4 88

(ppm), C2,3,5 71 (ppm), and C6 65 (ppm). Lignin, due to its heterogeneous nature,

produces a broad peak at 150 (ppm) (Boonstra et al. 1996). The NMR spectra of the

Wood OH + O

C

C

O

O

CH3

CH3

Wood O C CH3

O

+ CH3 C OH

O

+Wood OH CH3 C

O

O CH CH2 Wood O C CH3

O

+ CH2 CH OH

CH OCH3

(a)

(b)K2CO3

600 800 1000 1200 1400

1600

1800

2000



AA- and VA-modified wood flour showed new peaks in the 21 (ppm) and 170 (ppm)

regions due to the methyl band and the carboyxlic group of the acetyl group, respectively.

Fig. 3. 13

C CP-MAS NMR spectra of unmodified (W), acetic anhydride-modified (24% WPG) (WAA), and vinyl acetate-modified (19% WPG) (WVA) Scots pine wood flour

Figure 4 shows the density of the produced WPC groups. Statistical analysis

indicated a difference between HDPE and W20. It is believed that the cellular structure of

wood partially collapses during injection molding due to the high pressure, which leads to

some increase in density. Especially for the injection-molded WPCs, similar results have

been reported (Stark et al. 2004). It should also be noted that after acetylation of wood

flour, the densities of the resulting composites were slightly reduced. The reason for this

reduction might be that acetylated wood flour contains less lignocellulosic material than

unmodified wood flour at a given weight % loading.

Figure 5 shows the tensile strengths of the HDPE, W20, AA20, and VA20

composites at 20 wt% wood flour loading. It is clear from the figure that the acetylation

of Scots pine wood flour had a positive effect on tensile strength. Acetylation substitutes

the wood cell wall hydroxyl groups with acetyl groups (see Fig. 1), altering the wood

surface in particular, making it more hydrophobic. Since now both AA20 or VA20 and

HDPE are hydrophobic, it is predicted that AA20 or VA20 might exhibit better compat-

ibility with HDPE than W20. This would result in an increase in tensile strength of the

composites as the fiber–matrix interaction is now possible to facilitate stress transfer from

2 3 3 Wavenumber cm

-1

Transmittance (%)

3440

1375

1242

W

WVA

WAA



HDPE to AA20 or VA20. It should also be noted that the addition of extractive-free

wood flour to the HDPE polymer matrix did not reduce the tensile strength.

The tensile modulus values of the HDPE, W20, AA20, and VA20 composites at

20 wt% wood flour loading are presented in Fig. 6. The tensile modulus of the HDPE

significantly increased with the addition of wood flour. This is usually explained by the

rule of mixture (Matuana and Balatinecz 1998; Mengeloglu and Karakus 2008). Wood

flours have a higher modulus than polymer matrixes, and as a result, their mixture

produces higher modulus values compared to the polymer itself. From the figure, it is

also seen that acetylation of Scots pine wood flour with VA did not affect tensile

modulus values; however, acetylation of wood flour with AA had a negative effect on the

tensile modulus property compared to unmodified wood flour-filled composites (W20).

HDPE W20 AA20 VA20

0,6

0,7

0,8

0,9

1,0

1,1

BB

De

nsity (

g/c

m3)

B

A

Fig. 4. Density of WPCs

HDPE W20 AA20 VA20

16

18

20

22

24

26

Te

nsile

Str

en

gth

(M

Pa

)

C

C

B

A

Fig. 5. Effect of acetylation on the tensile strength (TS) values of WPCs



Figure 7 shows flexural strength values for HDPE, W20, AA20, and VA20

composites at 20 wt% wood flour loading. It is clear from the Figure that the acetylation

of Scots pine wood flour had a positive effect on the flexural strength. It should be noted

that injection-molded WPC even without modification might give slightly higher flexural

strength values (Gosselin et al. 2006). After acetylation of wood flour, a further increase

in flexural strength was observed.

HDPE W20 AA20 VA20

0

50

100

150

200

250

300

350

400

450

500

550

600

650

Te

nsile

Mo

du

lus (

MP

a)

C

AA

B

Fig. 6. Effect of acetylation on the tensile modulus values of WPCs

HDPE W20 AA20 VA20

20

22

24

26

28

30

32

34

36

38

Be

nd

ing

Str

en

gth

(M

Pa

)

D

C

B

A

Fig. 7. Effect of acetylation on the flexural strength values of WPCs

The flexural modulus (Fig. 8), similar to the tensile modulus, was increased with

the addition of wood flour to the polymer matrix. Acetylation of wood flour with AA

slightly reduced these values. A possible reason for reduction of the flexural modulus that

AA modified samples (24% WPG) contains 5.8% less lignocellulosic materials than



unmodified wood flour at 20 wt% loading. Because after acetylation treatment, the wood

flour gains weight as a result of the presence of bonded acetyl groups in the cell wall, a

given weight of modified wood flour therefore contain fewer lignocellulosic materials

than unmodified one (Hill 2006).

Figure 9 shows elongation at break values of the HDPE, W20, AA20, and VA20

composites at 20 wt% wood flour loading. It is clear from the Figure that compared to

the unfilled HDPE (around 500%), elongation at break values of the composites were

significantly lower (around 10%). It is well established that the addition of wood flour to

the polymer matrix dramatically reduces the elongation at break values. It is also reported

that higher modulus values usually results in lower elongation at break values for this

type of composite (Mengeloglu and Karakus 2008).

HDPE W20 AA20 VA20

600

800

1000

1200

1400

1600

Be

nd

ing

Mo

du

lus (

MP

a)

C

ABB

A

Fig. 8. Effect of acetylation on the flexural modulus values (b) of WPCs

HDPE W20 AA20 VA20

0

5

10

15

480

490

500

510

Elo

ng

atio

n a

t B

rea

k (

%)

A

B BB

Fig. 9. Effect of acetylation on the elongation at break values of WPCs



Figure 10 presents the impact strength values of the samples. Impact strengths

were significantly reduced with the wood flour addition.

HDPE W20 AA20 VA20

0

20

40

60

80

250

300

Imp

act S

tre

ng

th (

J/m

)

A

B B

C

Fig. 10. Effect of acetylation on the impact strength (IS) values of WPCs

Acetylation had no effect on the impact strength. Reduction of the impact strength

was reported by Mengeloglu and Karakus (2008) when the commercial coupling agent

MAPE was used. This is explained by the fact that better bonding between the polymer

and wood flour changed the mode of failure from fiber pullout to fiber breakage. This

study was the first time VA was used, and it improved the tensile and flexural strengths

as well as the modulus, but not the impact strength. Further studies will be conducted

with VA modifications.

The fracture mode of the unmodified wood flour composite (W20) is shown in

Fig. 11a. As shown for the freeze-fractured surface, wood fibers are debonded from and

pulled out of the cavitated HDPE matrix, which indicates poor interfacial adhesion. The

SEM micrographs shown in Figs. 11b and 11c display acetylated wood flour with AA or

VA embedded in the HDPE matrix. The acetylated wood flour is coated with a polymer

layer and shows good interaction between the two materials.

Fig. 11. SEM micrograph of freeze-fractured surface of a) W20 composite (untreated wood flour- reinforced HDPE) illustrating cavitated HDPE matrix, b) AA20 composite (acetic anhydride-modified wood flour-reinforced (24% WPG) HDPE) showing covered wood flour material connecting the wood flour to the matrix, and c) VA20 composite (vinyl acetate-modified wood flour-reinforced (19% WPG) HDPE) showing covered wood flour material connecting the wood flour to the matrix.

W

AA



The composites reinforced with untreated wood flour and those treated by

acetylation (AA or VA) were evaluated by thermogravimetric analysis (TGA) for

verification of the thermal characteristics of the materials. This characterization allowed

for evaluation of the influence of acetylation treatment of wood flour in the matrix.

Figures 12 to 14 show TG curves of W20 (untreated wood flour-reinforced HDPE 20

wt%), AA20 (acetic anhydride-modified wood flour-reinforced HDPE 20 wt%), and

VA20 (vinyl acetate-modified wood flour-reinforced HDPE 20wt%) composites.

For the W20 composite (Fig. 12), three stages of thermal decomposition were

noticed: 25 to 150C, 150C to 400C, and, 400 to 525C, which were denoted as region

I, region II, and region III, respectively. Complete decomposition of the composite

occurred at 525C. The first stage is related to the release of water or moisture. The

second stage is related to the degradation of the wood components of the composite. The

third region is related to degradation of HDPE. Similarly, three stages of behavior were

also observed with acetylated wood flour-reinforced HDPE composites (Figs. 13 and 14).

For pyrolysis of the untreated wood flour in the HDPE matrix, the active

decomposition temperature of the wood flour was determined to be 310C. For the

acetylated wood flour in the HDPE matrix, the main decomposition temperatures of the

acetylated wood flour shifted to higher values: 340 and 370C for AA (Fig. 13) and VA

(Fig. 14) modified samples, respectively. It is believed that acetylation with VA enhances

the thermal stability properties of lignocellulosic materials.

Fig. 12. TGA curves of W20 wood polymer composite



Fig. 13. TGA curves of AA20 wood polymer composite

Fig. 14. TGA curves of VA20 wood polymer composite

It was reported in an earlier study that after acetylation of hemp fibers with VA

(15% WPG) the active decomposition temperature shifted from 376°C to 392°C (Özmen

2012). The W20, AA20, and VA20 polymer composites consisted of around 6.9%, 5.3%,

and 3.4% final ash content, respectively.

CONCLUSIONS 1. Results of AA and VA modification yielded 24% and 19% WPG, respectively. These

results were obtained with AA and VA modification under the same reaction

conditions.

2. The acetylation of Scots pine flour in a polymer matrix in general results in

improvement in tensile strength and tensile and flexural modulus, as well as flexural

strength properties compared to neat composite and extracted unmodified wood flour

reinforced-composite.

3. The composites reinforced with VA acetylated wood flour exhibited better

mechanical properties than the composites with AA acetylated wood flour. It would



be of great interest to investigate the effect of WPG levels of VA modification on the

properties of composites in future studies.

4. The addition of wood flour or acetylated wood flour significantly reduces the impact

strength compared to neat composites.

5. The composites reinforced with acetylated wood flour presented higher thermal

stability than the composites reinforced with untreated wood flour. With acetylation,

the maximum weight loss temperature of the wood polymer composites was

increased by 30C (for AA-modified) and 60°C (for VA-modified) when compared

with untreated wood flour. Acetylated wood flour with VA exhibited better thermal

stability than acetylated wood flour with AA.

ACKNOWLEDGMENTS

This paper has been carried out with financial support from the KSU, BAP 2011-

3-YLS29.

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Article submitted: November 5, 2012; Peer review completed: December 8, 2012;

Revised version received and accepted: December 13, 2012; Published: December 14,

2012.

Documents

PEER-REVIEWED ARTICLE bioresources · thermoplastic (or many thermoset) polymers are hydrophobic. As a result of this divergent behavior, the adhesion between plant fibers and polymers